Time-reversal block transmit diversity system for channels with intersymbol interference and method

ABSTRACT

A method for diversity transmission and reception for channels with intersymbol interference is created. With this method one can transmit from two or more antennas in such a way that a receiver with one or more antennas can benefit from the diversity offered by the difference in channels from the transmit antennas to the receiver antenna(s). The way the transmission and reception is organized makes it relatively simple to in the receiver detect the transmitted symbols despite intersymbol interference in the channel. Due to the increased diversity experienced by the receiver the average power level required at the receiver is reduced which can be used to increase the capacity or coverage of a wireless network and/or reduce the required transmitted power.

RELATED APPLICATION

This application is a continuation application of pending U.S. Ser. No.09/833,543, filed Apr. 11, 2001, which claims the benefit of U.S.Provisional Application No. 60/197,306 filed Apr. 14, 2000; each ofwhich applications are hereby incorporated by reference.

FIELD OF THE INVENTION

The present invention relates, in general, to techniques that reduce theeffects of fading in wireless communication systems, and moreparticularly to techniques which reduce the variation in signal strengthof the received signal, while still effectively handling intersymbolinterference.

BACKGROUND OF THE INVENTION

Sending a signal in the form of symbols transmitted at radio frequenciesis one way of sending information. Several problems exist with thisapproach. A wireless communication channel typically experiences fading,i.e. the received signal strength varies with time and the position ofthe receiver and/or the transmitter. Further, a wireless communicationchannel often suffers from intersymbol interference, i.e.super-positioning of delayed versions of the transmitted symbolsequence. Intersymbol interference arises, for example, when a receiverpicks up delayed versions of a single transmission. Buildings, mountainsand other objects create delayed copies of a signal when a transmissionreflects off the surface of the object and arrives at the receiver laterthan a version having fewer or no reflections before arriving at thereceiver. The spread in time between the different copies of a signal iscalled the delay spread. The delay spread results in multiple overlaidcopies of the signal with different amplitudes, phases and delays. Themultiple copies interfere with the intended signal transmission,becoming noise and causing signal disruption.

Another problem with wireless communication is that the variation insignal strength at a receiver typically requires the system to bedesigned to transmit with higher power than would be necessary if thesignal strength was constant, or if it varied less. This typicallyreduces the capacity of the system.

S. M. Alamouti (1, 2) proposes a method of overcoming this limitation.He provides a two-branch transmit diversity scheme in which two transmitantennas and one receive antenna provide the same diversity as can beachieved with one transmit antenna and two receive antennas. This meansthat the same reduction in the variation of the quality of the receivedsignal that can be achieved with two receive antennas can instead berealized with two transmit antennas. In the case of a cellular wirelesssystem with base stations and subscriber units, the variability on boththe uplink and the downlink can be combated with only multiple antennasat the base station, rather than at the subscriber unit, where it iscostly and cumbersome to have multiple antennas.

A problem with the S. M. Alamouti two-branch transmit diversity schemeis that it does not effectively handle intersymbol interference in thechannel. When a channel suffers from intersymbol interference, multipleversions of the original symbol sequence are received with differentdelays making the detection of the symbol sequence more difficult.Intersymbol interference can be caused by multiple propagation pathswith different delays or by the use of transmission pulse shaping thatextends over more than one symbol interval, or by the receive filter.The transmission pulse shaping and the receive filter is considered tobe part of the channel. When there is intersymbol interference in thechannel, the S. M. Alamouti scheme loses some of its good properties.More specifically, because of the intersymbol interference in thechannel the receiver cannot be realized in the simple form described byS. M. Alamouti. Instead a considerably more complex receiver is berequired. This dramatically reduces the usefulness of the scheme forchannels with intersymbol interference.

What is needed is a system and method of transmit diversity that enablesa transmitter to provide a better signal with less power while stillhandling intersymbol interference effectively with a relatively simplereceiver.

SUMMARY OF THE INVENTION

The invention overcomes the identified limitations and provides a systemand method for transmit diversity in channels with intersymbolinterference. We call the method time-reversal space-time block coding.The system and method reduces the variability of the quality of thesignal received by the receiver with a relatively simple receiveralgorithm, even for channels with intersymbol interference. Applyingtransmit delay diversity in two or more groups of antennas is anotherembodiment of the invention which further increases the number ofchannels used in the delay diversity scheme, and further reduces thevariability in the quality of the received signal.

The invention relates to a method of reducing the variability in thesignal level applicable to channels with intersymbol interference in asystem for processing and transmitting a signal where the signalcomprises a plurality of symbols. The system comprises a first and asecond spaced antenna coupled to a transmitter. In one example, themethod of reducing the variability in the signal level comprises thefollowing steps. Divide the symbols of the signal into a first and asecond symbol stream wherein the first and second symbol streams eachhave at least two symbols. Divide a transmission frame into a first anda second transmission block. Transmit the first symbol stream from thefirst antenna during the first transmission block and transmit thesecond symbol stream from the second antenna during the secondtransmission block. Time reverse, take the complex conjugate form of andnegate the second symbol stream. Time reverse and take the complexconjugate form of the first symbol stream. Transmit from the firstantenna during the second transmission block the second symbol stream inthe time reversed, complex conjugate and negated form, and transmit fromthe second antenna the first symbol stream in the time reversed andcomplex conjugate form.

The invention, in another embodiment, relates to a method oftransmitting a signal of the type comprising a sequence of symbols overspaced antennas, or antennas of different polarization, to reduce fadingand intersymbol interference, comprising the steps of: (1) dividing thesequence of symbols into two sequences, (2) dividing the transmissionframe into two blocks, (3) processing the symbols in said two sequencesso that some of the symbols in at least one of the sequences are timereversed, some of the symbols in at least one of the sequences arecomplex conjugated, some of the symbols in at least one of the sequencesare negated, and, (4) during one of the blocks of the transmissionframe, applying one processed symbol sequence to a first antenna and theother processed signal sequence to a second antenna, and during theother block of the transmission frame applying the other processedsymbol sequence to the first antenna and the one processed symbolsequence to the second antenna.

In a further embodiment, the invention relates to a method of receivingsymbol sequences transmitted in transmission frames having two blocksover spaced antennas, or antennas with different polarization in whichthe symbol sequence which is transmitted is divided into two sequences,some of the symbols in at least one of the two sequences are timereversed, some are complex conjugated and some are negated and, duringone of the blocks of the transmission frame, one processed symbolsequence is transmitted over the first antenna, and the other over thesecond antenna and, during the other block, the other symbol sequence istransmitted over the second antenna, and the one over the secondantenna. Said receiver receiving the symbol streams in the first andsecond blocks of the transmission frames and time reversing and takingthe complex conjugate form of the symbol stream in the second block andfiltering the symbol stream in the first block and the time reversedcomplex conjugate of the symbol streams in the second block to formdecoupled outputs.

The principles of this invention are also applicable to arrangementswith more than one receive antenna. The multiple receive antennas canfor example be used to combine the signal in order to improve the signalto noise ratio in the signal and suppress interference. Thegeneralization of the receiver processing to more than one receivingantenna can be done with well known methods.

DESCRIPTION OF THE DRAWINGS

The foregoing and other objects of the present invention will be moreclearly understood from the following description when read inconnection with the accompanying drawings in which:

FIG. 1 is a schematic diagram of a time-reversal block transmitdiversity system in accordance with the present invention.

FIG. 2 is a channel model for two transmit antennas and a receiveantenna.

FIG. 3 illustrates the received signal after complex conjugation ofr′₂(t) in the transmit diversity scheme for channels without symbolinterference.

FIG. 4 illustrates the received signal after manipulation in thetransmit diversity scheme for channels with intersymbol interference.

FIG. 5 is a schematic view of the transmission in the transmit diversityscheme for channels with intersymbol interference.

FIG. 6 is a schematic view of the symbol detection when using thetransmit diversity scheme for channels with intersymbol interference.

FIG. 7 illustrates the configuration of the training data and datatransmitted by the antennas.

GENERAL DESCRIPTION OF INVENTION

The invention will be generally described with reference to FIG. 1. Asymbol stream d(t) to be transmitted during a frame in the communicationsystem is fed into a space-time encoder. The space-time encoder dividesthe symbol stream d(t) into two symbol streams, d₁(t) and d₂(t), eachcontaining half the symbols. The transmission frame is also divided intotwo blocks. The space-time encoder provides input to two radiotransmitters 13 and 14 connected to two antennas 11 and 12. In thetransmitters 13 and 14, the digital signals from the space-time encoderare converted to analog signals via an analog-to-digital converter andupconverted to radio frequency. In one embodiment of the invention thespace-time encoder transmits symbol stream d₁(t) from antenna 11 duringa first block of the transmission frame and transmits symbol streamd₂(t) from the antenna 12. In a second block of the transmission frame,the space-time encoder transmits symbol stream d₂(t) time-reversed,complex conjugated and negated from antenna 11 and symbol stream d₁(t)is transmitted time-reversed and complex conjugated from antenna 12.

In the receiver, the signal is received by an antenna 16 anddownconverted to baseband and digitized using well known methods. Thedigital signal is then fed into a space-time decoupler 17 and a channelestimator 18. Preferably some of the transmitted symbols are known. Thechannel estimator can then use the knowledge of these symbols value toestimate the channels between the two transmit antennas and thereceiving antenna. These channel estimates are fed to the space-timedecoupler which filters the received signal in such a way that twodecoupled outputs, z₁(t) and z₂(t), are formed. These outputs aredecoupled in the sense that z₁(t) depends on d₁(t) but not on d₂(t) andz₂(t) depends on d₂(t) but not on d₁(t). The signals z₁(t) and z₂(t)still suffer from intersymbol interference due to the delay spread inthe propagation channel and/or partial response modulation in thetransmitter and/or delay spread in the receive filter. However, sincethe signals z₁(t) and z₂(t) each only depend on one stream of symbolsand not two, the estimation of the symbol streams d₁(t) and d₂(t) ismuch simpler than if z, (t) and z₂(t) would depend on two symbolsstreams each. This is an important part of this invention. Theequalization of the intersymbol interference in z₁(t) and z₂(t) todetect d₁(t) and d₂(t), respectively, can be performed with a maximumlikelihood sequence estimator. Other equalizers or detectors that can beused are for example linear equalizers and decision feedback equalizersand different variations of all of these detectors. Detectors 21 and 22can use the channel estimates from the channel estimator for the purposeof their tuning. After the symbol streams d₁(t) and d₂(t) have beendetected they are combined at stream combiner 23 to form an estimate ofthe originally transmitted symbol stream, d(t).

It is possible to make simple modifications and enhancements to themethod presented here without departing from the spirit of thisinvention. For example, it is possible to have different arrangements ofthe symbols in different number of streams and it is possible to dividea transmission frame into a different number of blocks, it is possibleto apply the time-reversal, complex conjugation and negation indifferent fashions to these blocks while still achieving the same finalgoal, namely transmitting from multiple antennas in such a way that thereceiver can recover the spatial diversity in a simple way even in thepresence of intersymbol interference in the channel.

It is also possible to formulate the detection of the symbolsdifferently in the receiver without departing from the spirit of theinvention. For example, in the receiver, one will want to utilize thefact that with proper combining and filtering or arrangement of thecomputations, the detection of the symbol streams d₁(t) and d₂(t)effectively decouple into two separate detections of the symbol streamsd₁(t) and d₂(t). In order to do this, the spirit of the invention mustbe utilized.

Another distinctive feature of this invention that can be realized, alsowith small modifications to how the transmission and reception is beingperformed, is the feature of transmitting in such a way and processingand filtering the received signal in such a way that multiple signalstreams are produced, each of them effectively being a filtered versionof a single symbol stream of symbol. This facilitates the mitigation ofthe intersymbol interference in the channel.

Another feature of this invention is how to signal over a channel suchthat the effective experienced channel is a time-reversed version of theactual channel. How this is performed is described in the detaileddescription of the invention. The ability to signal such that thetime-reversed channel is experienced is important in order to handleintersymbol interference effectively when transmitting from multipleantennas to take advantage of diversity.

DETAILED DESCRIPTION OF THE INVENTION

Throughout this description, we will consider discrete-time channelmodels and detectors. A discrete-time filter will be represented as apolynomial in the unit delay operator, q⁻¹, as exemplified below:$\begin{matrix}{{v(t)} = {{a\left( q^{- 1} \right)}{u(t)}}} \\{= {\left( {a_{0} + {a_{1}q^{- 1}} + \cdots + {a_{na}q^{- {na}}}} \right){u(t)}}} \\{{= {{a_{0}{u(t)}} + {a_{1}{u\left( {t - 1} \right)}} + \cdots + {a_{na}{u\left( {t - {na}} \right)}}}},}\end{matrix}$where na is the order of the polynomial a(q⁻¹), representing a filterwith na+1 taps. The discrete time is denoted with the discrete variablet. Note that filters may also be non-causal and have terms with powersof the unit advance operator q.

Multiple-input-single-output (MISO) filters will be represented aspolynomial row vectors, and single-input-multiple-output (SIMO) filterswill be represented as polynomial column vectors.Multiple-input-multiple-output (MIMO) filters will be represented aspolynomial matrices.

The complex conjugate of a filter a(q⁻¹) is defined as(a(q ⁻¹))*=^(Δ) a*(q)=a* ₀ +a* ₁ q+ . . . +a* _(na) q ^(na)  (1)Note that the resulting filter is anti-causal.

Correspondingly, the complex conjugate transpose of a MISO, SIMO or MIMOfilter is the transpose of the filter with all filter elements complexconjugated according to (1).

The discrete-time model of a channel with two transmit antennas and onereceive antenna is given by $\begin{matrix}\begin{matrix}{{y(t)} = {{{h_{1}\left( q^{- 1} \right)}{d_{1}(t)}} + {{h_{2}\left( q^{- 1} \right)}{d_{2}(t)}} + {n(t)}}} \\{= {{\left( {h_{10} + \ldots + {h_{1,{{nh}_{1} - 1}}q^{{{- {nh}_{1}}1} + 1}}} \right){d_{1}(t)}} +}} \\{{{\left( {h_{20} + \ldots + {h_{2,{{nh}_{2} - 1}}q^{{- {nh}_{2}} + 1}}} \right){d_{2}(t)}} + {n(t)}},}\end{matrix} & (2)\end{matrix}$where (2), y(t) is the received signal, d₁(t) and d₂(t) are the symbolsequences transmitted from antenna 11 and antenna 12 respectively, andh₁(q⁻¹) and h₂(q⁻¹) are the channels for antenna 11 and antenna 12respectively. The additive noise is modeled by n(t). We will in thisdescription assume that the noise is white with variance σ_(n) ². SeeFIG. 2. The channel is assumed to be fading but to be approximatelystationary over a block of symbols. Note that the pulse shape used inthe modulation and the receive filter is part of the overall channelmodeled in (2).

The invention here is described in terms of complex baseband processingin digital form. It is assumed that there are digital-to-analogconverters that convert the digital baseband signals to be transmittedinto analog signals that in turn are upconverted to radio frequency withradios using well known methods. These radio signals are thentransmitted from antennas over the radio channel. Correspondingly it isassumed that the radio signals are received by antenna(s) anddownconverted with radios using well known methods to an analog signalsat baseband. This signal is then sampled and converted into a complexdigital baseband signals using analog-to-digital converters.

A. Channel Without Intersymbol Interference

Let us for a moment assume that the channel has no delay spread and thatwe do not use partial response modulation. Thus, without any intersymbolinterference the channels have only a single tap each, i.e.h ₁(q ⁻¹)=h ₁ and h ₂(q ⁻¹)=h ₂  (3)

In the scheme presented by Alamouti in [1, 2], the original symbolstream, d(t), is divided into two separate symbol streams, d₁(t) andd₂(t). These two symbol streams are then transmitted on antenna 11 andantenna 12 such that every “even” sample the signal,r ₁(t)=h ₁ d ₁(t)+h ₂ d ₂(t)+n ₁(t)  (4)is received. That is, d₁(t) is transmitted from antenna 111 and d₂(t) istransmitted from antenna 12. The noise n₁(t) represents thecorresponding even noise samples. Every “odd” sample the symbol streamsare transmitted such that the signal,r′ ₂(t)=h ₂ d* ₁(t)−h ₁ d* ₂(t)+n′ ₂(t)  (5)is received at the receiver. That is, d*₁(t) is transmitted from antenna12 and −d*₂(t) is transmitted from antenna 11. The noise n′₂(t)represents the corresponding odd noise samples. The received odd signalsamples are then complex conjugated giving the signalr ₂(t)=(r′ ₂(t))*=h* ₂ d ₁(t)−h* ₁ d ₂(t)+n ₂(t)  (6)

If we introduce the vectors $\begin{matrix}{{r = \begin{bmatrix}{r_{1}(t)} \\{r_{2}(t)}\end{bmatrix}},{{d(t)}\begin{bmatrix}{d_{1}(t)} \\{d_{2}(t)}\end{bmatrix}},} & (7) \\{{{n(t)} = {\begin{bmatrix}{n_{1}(t)} \\{n_{2}(t)}\end{bmatrix} = \begin{bmatrix}{n_{1}(t)} \\\left( {n_{2}^{\prime}(t)} \right)^{*}\end{bmatrix}}}{{and}\quad{the}\quad{matrix}}} & (8) \\{{H = \begin{bmatrix}h_{1} & h_{2} \\h_{2}^{*} & {- h_{1}^{*}}\end{bmatrix}},} & (9)\end{matrix}$we can express the transmission from the two symbol streams d₁(t) andd₂(t) to the received sequences r₁(t) and r₂(t) asr(t)=Hd+n(t)  (10)

Note that the “channel matrix” H is orthogonal such thatH ^(H) H=(|h ₁|² +|h ₂|²)I  (11)

In [1, 2], Alamouti proposes to multiply r(t) with H^(H) in the receiverto obtain the signal $\begin{matrix}{\begin{matrix}{{z(t)} = {{H^{H}{r(t)}} = {{H^{H}H\quad{d(t)}} + {H^{H}{n(t)}}}}} \\{{= {{\left( {{h_{1}}^{2} + {h_{2}}^{2}} \right){d(t)}} + {v(t)}}},}\end{matrix}{where}} & (12) \\{{v(t)} = {\begin{bmatrix}{v_{1}(t)} \\{v_{2}(t)}\end{bmatrix} = {H^{H}{{n(t)}.}}}} & (13)\end{matrix}$Note that H^(H) is in fact the matched filter and that z(t) in (12) isthe matched filter output.

Using the components of z(t)=[z₁(t)z₂(t)]^(T), we can express (12) asz ₁(t)=(|h ₁|² +|h ₂|²)d ₁(t)+v ₁(t)  (14)z ₂(t)=(|h ₁|² +|h ₂|²)d ₂(t)+v ₂(t)  (15)

Using (11), we can compute the covariance of the noise vectorv(t)=[v₁(t) v₂(t)]^(T) as $\begin{matrix}\begin{matrix}{R_{vv} = {E\left\lbrack {{v(t)}{v^{H}(t)}} \right\rbrack}} \\{= {H^{H}R_{nn}H}} \\{{= {{\sigma_{n}^{2}H^{H}H} = {{\sigma_{n}^{2}\left( {{h_{1}}^{2} + {h_{2}}^{2}} \right)}I}}},}\end{matrix} & (16)\end{matrix}$In the third equality we have used the fact that n(t) is a white vectornoise sequence with the covariance R_(nn)=σ_(n) ²I. Since R_(vv) isdiagonal, v₁(t) and v₂(t) are uncorrelated.

The matched filter outputs, z₁(t) and z₂(t), can obviously after properscaling and slicing be used independently to estimate the transmittedsymbols d₁(t) and d₂(t) respectively. The reason for this simplifieddecoupled detection is of course that the channel matrix H is orthogonaland thus H^(H)H is diagonal. The detection of the two symbol streams,d₁(t) and d₂(t), thus decouples making the detection very easy. Further,since v₁(t) and v₂(t) are uncorrelated, no gain can be achieved by jointdetection of the two symbol streams.

If we had one transmit antenna and two receive antennas thecorresponding matched filter output would bez(t)=(|h ₁|² +|h ₂|²)d(t)+v(t)  (17)We can thus see that the matched filter output in (12), using twotransmit and one receive antenna, experiences the same diversity benefitas when using one transmit and two receive antennas¹. This was shown byAlamouti [1, 2].¹However, unless we know the channel when transmitting, we cannotachieve the gain from coherent combining.

The received signals after complex conjugation of r′₂(t) in the transmitdiversity scheme for channel without intersymbol interference is shownin FIG. 3.

B. Channel with Intersymbol Interference

Let us now return to our original channel model in (2) with intersymbolinterference. With the proper substitutions and manipulations we canderive the counterpart of the scheme by Alamouti [1, 2] for a channelwith intersymbol interference. This derivation is however not trivial.

Assume that we transmit in such a way that the received signal has theform $\begin{matrix}{{{r(t)} = {{{H\left( {q,q^{- 1}} \right)}{d(t)}} + {n(t)}}},{where}} & (18) \\{H = {\begin{bmatrix}{h_{1}\left( q^{- 1} \right)} & {h_{2}\left( q^{- 1} \right)} \\{h_{2}^{*}(q)} & {- {h_{1}^{*}(q)}}\end{bmatrix}.}} & (19)\end{matrix}$The noise vector n(t) is the noise after the necessary manipulation inthe receiver which will be explained below. It will be white with thecovariance R_(nn)=σ_(n) ²I. Note that the channels h*₂(q) and h*₁(q)have complex conjugated coefficients and are time reversed and thusanti-causal. We will see below how this signalling can be achieved.

The polynomial channel matrix H(q, q⁻¹) is also here orthogonal in thesense thatH ^(H)(q,q ⁻¹)H(q,q ⁻¹)=(h* ₁(q)h* ₁(q ⁻¹)+h* ₂(q)h ₂(q ⁻¹))I.

In the receiver we now filter this signal with the matched filterH^(H)(q, q⁻¹). The output from the matched filter is then given by$\begin{matrix}{\begin{matrix}{{z(t)} = {{{H^{H}\left( {q,q^{- 1}} \right)}{H\left( {q,q^{- 1}} \right)}{d(t)}} + {{H^{H}\left( {q,q^{- 1}} \right)}{n(t)}}}} \\{{= {{\left( {{{h_{1}^{*}(q)}{h_{1}\left( q^{- 1} \right)}} + {{h_{2}^{*}(q)}{h_{2}\left( q^{- 1} \right)}}} \right){d(t)}} + {v(t)}}},}\end{matrix}{where}} & (20) \\{{v(t)} = {\begin{bmatrix}{v_{1}(t)} \\{v_{2}(t)}\end{bmatrix} = {{H^{H}\left( {q,q^{- 1}} \right)}{{n(t)}.}}}} & (21)\end{matrix}$Using the components of z(t)=[z₁(t)z₂(t)]^(T), we can express (20) asz ₁(t)=(h* ₁(q)h ₁(q ⁻¹)+h* ₂(q)h ₂(q ⁻¹))d ₁(t)+v ₁(t)  (22)z ₂(t)=(h* ₁(q)h ₁(q ⁻¹)+h* ₂(q)h ₂(q ⁻¹))d ₂(t)+v ₂(t)  (23)Similar to (16), the noise sequences v₁(t) and v₂(t) are uncorrelated asthe spectrm² of v(t) given by $\begin{matrix}\begin{matrix}{{R_{vv}\left( {q,q^{- 1}} \right)} = {\sum\limits_{m = {- \infty}}^{\infty}{{E\left\lbrack {{v(t)}{v^{H}\left( {t - m} \right)}} \right\rbrack}q^{- m}}}} \\{= {{H^{H}\left( {q,q^{- 1}} \right)}{R_{nn}\left( {q,q^{- 1}} \right)}{H\left( {q,q^{- 1}} \right)}}} \\{= {\sigma_{n}^{2}{H^{H}\left( {q,q^{- 1}} \right)}{H\left( {q,q^{- 1}} \right)}}} \\{= {{\sigma_{n}^{2}\left( {{{h_{1}^{*}(q)}{h_{1}\left( q^{- 1} \right)}} + {{h_{2}^{*}(q)}{h_{2}\left( q^{- 1} \right)}}} \right)}I}}\end{matrix} & (24)\end{matrix}$has no cross terms between v₁(t) and v₂ (t). In the third equality wehave used the fact that n(t) is a white vector noise sequence withR_(nn)(q, q⁻¹)=σ_(n) ²I.² We have here replaced z in the spectrum with q to simplify thenotation. See [3].

The problem of detecting the symbol streams d₁(t) and d₂(t) thusdecouples. Furthermore, the channel after matched filtering is the sameas one would obtain when using one transmit antenna and two receiveantennas. This scheme thus, similar to the case without intersymbolinterference, obtains the same diversity benefit as one can achieveusing one transmit and two receive antennas. It thus achieves fulldiversity. The intersymbol interference of course still has to behandled by an equalizer. The output from the matched filter is howeverexactly the signal to be processed by a maximum likelihood sequenceestimator utilizing the matched filter metric. See for example [4, 3].Again it should be noted that, as for the case without intersymbolinterference, the gain from coherent combining obtained when using tworeceive antennas is not reproduced when using two transmit antennas andone receive antenna.

When using an MLSE, the estimated symbol sequence, {circumflex over(d)}₁(t), will be the symbol sequence that maximizes the recursivelydefined matched filter metric [4, 3] $\begin{matrix}{{\mu_{MF}(t)} = {{\mu_{MF}\left( {t - 1} \right)} + {{Re}{\left\{ {{d_{1}^{*}(t)}\left( {{2{z_{1}(t)}} - {\gamma_{0}{d_{1}(t)}} - {2{\sum\limits_{m = 1}^{n\quad\gamma}{\gamma_{m}{d_{1}\left( {t - m} \right)}}}}} \right)} \right\}.}}}} & (25)\end{matrix}$In (25), γ_(k) are the coefficients of the double sided complexconjugate symmetric metric polynomialγ(q,q ⁻¹)=γ*_(nγ) q ^(nγ)+ . . . +γ₀+ . . . +γ_(nγ) q ^(−nγ) =h* ₁(q)h₁(q ⁻¹)+h* ₂(q)h ₂(q ⁻¹)  (26)Preferably, the maximizing sequence is found using the Viterbialgorithm³ [5]. The estimated symbol sequence {circumflex over (d)}₂(t)is similarly formed by maximizing the corresponding metric utilizing thesecond component, z₂(t), of z(t).³ In order to save computations, various suboptimal schemes can also beused.

In order for the symbol detector in the receiver to work properly it hasto be adapted to the channel. One way of doing this is to estimate thechannel using the known transmitted symbols. This estimation of thechannel can be performed in many different ways using well knownmethods. One method of estimating the channel is to estimate thepolynomials h₁(q⁻¹) and h₂(q⁻¹) that best model the part of the receivedsignal that correspond to the known transmitted symbols. This approachis well known and is only one of the examples of how channel estimationcan be performed. In the symbol detector described here the maximumlikelihood sequence detector using the metric in (25), one need toestimate the channel polynomials h₁(q⁻¹) and h₂(q⁻¹) and use them toform the metric polynomial in (26), whos coefficients are used in themaximum likelihood sequence detector metric in (25).

C. Anti-Causal Signalling

Consider the components r₁(t) and r₂ (t) of the vector signalr(t)=[r₁(t) r₂ (t)]^(T):r ₁(t)=h ₁(q ⁻¹)d ₁(t)+h ₂(q ⁻¹)d ₂(t)+n ₁(t)  (27)r ₂(t)=h* ₂(q)d ₁(t)−h* ₁(q)d ₂(t)+n ₂(t)  (28)

To receive r₁(t) we simply transmit the symbol stream d₁(t) from antenna11 and symbol stream d₂(t) from antenna 12. However, since there isintersymbol interference in the channel we cannot transmit such as toreceive r₁(t) and r₂(t) in alternating symbol intervals. We have totransmit such as to receive a longer sequence of r₁(t), and a longersequence of r₂(t). We will describe this in more detail below.

Achieving r₂(t) at the receiver is less straightforward but nonethelesspossible. Consider the two symbol streams d₁(t) and d₂(t). Let us choosetheir length to be N+1. Time reverse these symbol streams to form thenew symbol streams{tilde over (d)} ₁(t)=d ₁(N−t), t=0, 1, . . . , N  (29){tilde over (d)} ₂(t)=d ₂(N−t), t=0, 1, . . . , N  (30)Now transmit −{tilde over (d)}*₂(t) from antenna 11 and {tilde over(d)}*₂(t) from antenna 12. The signal at the receiver will then ber′ ₂(t)=h ₂(q ⁻¹){tilde over (d)}* ₁(t)−h ₁(q ⁻¹){tilde over (d)}*₂(t)+n(t)  (31)By time reversing r′₂(t) in (31) and complex conjugating it we obtainthe signal(r′ ₂(N−t))*=h* ₂(q)d ₁(t)−h* ₁(q)d ₂(t)+n ₂(t)  (32)where we have denoted n*(N−t) with n₂(t). Note that the signal in (32)is is exactly the desired signal r₂(t) in (28).

The received signal after manipulation in the transmit diversity schemefor channels with intersymbol interference is shown in FIG. 4.

The transmit diversity scheme can thus be summarized as follows. Dividea sequence of symbols, d(t), t=0, 1, . . . , 2N+2, into two sequences,d₁(t), t=0, 1, 2, . . . , N and d₂(t), t=0, 1, 2, . . . , N. Thisdivision of the symbol sequence d(t) into two symbol sequence can bemade more or less arbitrary as long as there is an equal amount ofsymbols in each sequence d₁(t) and d₂(t) and the correlation betweensymbols in the sequences close to each other is not significantlyeffected. Also divide a transmission frame into two blocks. During thefirst block of the frame, transmit the sequence d₁(t) from antenna oneand the sequence d₂(t) from antenna two. During the second block of theframe, transmit d₂(t) time reversed and complex conjugated from antenna11 and transmit d₁(t) time reversed, complex conjugated and negated fromantenna 12. The transmission procedure is depicted in FIG. 5.

On the receive side, during the first block of the frame, the samplesare collected to form the sequence r₁(t) and during the second block ofthe frame the samples are collected and the sequence is complexconjugated and time reversed in order to form the sequence r₂(t). Thesequences r₁(t) and r₂(t) are then fed into the MIMO matched filterH^(H)(q, q⁻¹) to form the decoupled outputs z₁(t) and z₂(t). Thesequences z₁(t) and z₂(t) are then used independently to estimate thetransmitted sequences d₁(t) and d₂(t). This detection can for example beperformed with a maximum likelihood sequence estimator. The receivesignal processing is schematically depicted in FIG. 6.

The way the symbols are transmitted and received, as described above, isa principal part of this invention. Especially important is the conceptof time reversing the symbol streams when they are transmitted in thesecond block of the frame, and in the receiver time reversing the signalreceived during the second block of the frame. These time reversaloperations is what enables the simple detection described in thisinvention. Without these time reversal operations, and the matchedfiltering described for the receiver, the detection of the two symbolstreams, d₁(t) and d₂(t) does not decouple. There are many variations asto how the transmission and the reception can be arranged to achievethis effect. We can change which symbol sequence is being negated, whichsymbol streams are being complex conjugated and which symbol streams arebeing time reversed. Apart from changing the way in which thetransmission is performed it will change the corresponding matchedfilter, H^(H)(q, q⁻¹), that is being applied in (20) in the receiver.All these are simple variations of this invention. The main principle isto arrange the transmission of the symbols in such a way that after theyhave passed through the channel and has been processed in the receiveras descibed in this invention, two outputs are produced that each dependonly on one of the sequences, d₁(t) or d₂(t), and also is easy toequalize as described in this invention using, for example, a maximumlikelihood sequence estimator.

A very important component of this invention is how to signal over achannel such that the effective experienced channel is a time-reversedversion of the actual channel. We here describe the principle for howthis can be achieved.

Assume that we have a time-discrete symbol stream d(t), t=1, 2, . . . ,N and a channel described by the polynomial h(q⁻¹). If we transmit thesymbol stream d(t) over the channel h(q⁻¹), sampling the received signalonce per symbol interval, the sampled output in the receiver, y(t), canbe expressed asy(t)=h(q ⁻¹)d(t)+n(t)  (33)where n(t) is a term representing noise plus interference. Let us nowassume that we want to form a signal, {tilde over (y)}(t), of the form{tilde over (y)}=(t) h(q)d(t)+v(t)  (34)where v(t) is another representation of noise and interference and h(q)is a time reversed version of h(q⁻¹), i.e. the delay operators, q⁻¹, inh(q⁻¹) are replaced by the advance operator q. In other words, ifh(q ⁻¹)=h ₀ +h ₁ q ⁻¹ + . . . +h _(nh) q ^(−nh)  (35)thenh(q)=h ₀ +h ₁ q+ . . . +h _(nh) q ^(nh)  (36)It is not trivial to signal with d(t) over h(q⁻¹) in such a way that{tilde over (y)}(t) is generated but it can be done as follows.

Take the symbol stream d(t) and time-reverse it to form thetime-reversed symbol stream{tilde over (d)}(t)=d(N+1−t), t=1, 2, . . . , N  (37)Transmit {tilde over (d)}(t) over the channel h(q⁻¹) such that thesignalx(t)=h(q ⁻¹){tilde over (d)}(t)+n′(t)  (38)is received. Time reverse the signal x(t) giving the desired signal{tilde over (y)}(t)=x(N+1−t), t=1, 2, . . . N  (39)Because {tilde over (y)}(t) is a time-reversed version of x(t) it can beexpressed as in equation (34) and therefore is the signal we desire.

Due to the intersymbol interference the signalling suffers from some“edge effects”. These can however be handled by insertion of knownsymbols in the beginning and end of each transmission block. Let usdefine the maximum delay in the channels asnh= ^(Δ) max(nh ₁ , nh ₂)  (40)The first nh samples of r₁(t) will thus not conform with (27) andsimilarly the last nh of r₂(t) will not conform with (28). The matchedfilter in the receiver (20), filters r₁(t) with h*₁(q) or h*₂(q) andr₂(t) with h₂(q⁻¹) or −h₁(q⁻¹). As a result the matched filter signal,z(t), will only conform with (20) when tε[nh+1, N−nh].

In the beginning and the end of each of the r₁- and r₂-blocks, nhsymbols can thus not be used in the simplified detection outlined inthis description. This is however not a big problem. We will in any caseneed some training symbols in order to estimate the channels h₁(q⁻¹) andh₂ (q⁻¹). We can thus put these training symbols in the beginning andthe end of each of the r₁- and r₂-blocks, or more precisely, in thebeginning and the end of the sequences d₁(t) and d₂(t), and thus also inthe beginning and end of the sequences −d*₂(N−t) and d*₁(N−t). Thetraining symbols at the end of the r₁-block and at the beginning of ther₂-block can always be combined to a longer training sequence. This isimportant since when estimating channels with intersymbol interference,the training sequence cannot be allowed to be too short. Further, extratraining symbols can be inserted in between the end of the r₁-block andbefore the begining of the r₂-block.

Also, if this scheme is used in the transmission from a base station,then the receiving subscriber can potentially combine the trainingsymbols at the end of an r₂-block with the beginning of an r₁-block toform yet another longer training sequence.

Note that all symbols, including the training symbols, transmitted inthe r₂-block are time reversed compared to the corresponding symbols inthe r₁-block. The training sequences in the r₂-block are thus timereversed compared to the the training sequences in the r₁-block. Thenumber of training symbols in the beginning and the end of the sequencesd₁(t) and d₂(t) has to be at least equal to the maximum expected delay,nh, in symbol periods. FIG. 7 shows the configuration of training dataand data. The upper row of data is transmitted from antenna 11 and thelower row is transmitted from antenna 12.

I. Combining With Transmit Delay Diversity

We will here call the method of transmit diversity described above‘time-reversal space-time block coding’. Since time-reversal space-timeblock coding can handle intersymbol interferference, we can combine itwith the well known method of transmit delay diversity [6]. In transmitdelay diversity artificial delay spread is introduced in the channel bytransmitting the same signal from two or more antennas with some delaybetween the transmissions from the different antennas. The delay betweenthe antenns would typically be of the order of a symbol interval. Thisartificially introduced delay spread in the channel introduces diversitythat can be exploited by the equalizer or sequence detector in thereceiver. We can now combine transmit delay diversity with time-reversalspace-time block coding as follows.

Let us divide a group of transmit antennas into two groups. Within therespective groups we use transmit delay diversity. We then view the twogroups as two different channels and apply time-reversal space-timeblock coding to them. We thereby double the initial diversity that thetransmit delay diversity achieved within each group. With more diversitythe received signal level will vary even less and even less receivedpower is required at the subscriber unit. This can be used to furtherincrease the range of the system or further increase the capacity asless power can be transmitted from the base station, thus creating lessinterference and thus allowing more users in the system.

The combination of time-reversal space-time block coding with transmitdelay diversity is a part of this invention.

A new transmit diversity scheme for channels with intersymbolinterference, causing intersymbol interference, has been described. Theintersymbol interference can be caused by partial response modulation orby delay spread in the propagation channel. This scheme shares many ofthe benefits of the transmit diversity scheme for channels withoutintersymbol interference presented in [1, 2]. It can however, as opposedto the scheme descriped in [1, 2] handle channels with intersymbolinterferense efficiently. This is very important as most practicalwireless communication channels have some intersymbol interference fromeither partial response modulation in the transmitter or from delayspread in the propagation channel or from filtering in the receiver, orfrom all of these effects.

The detection of the symbol streams are decoupled, avoiding anunnecessarily complex detector. The scheme also achieves the samediversity benefit with two transmit antennas and one receive antenna ascan be achieved with one transmit antenna and two receive antennas. Thechannel is required to be approximately stationary over a block ofsymbols. The size of this block is a design parameter.

Note that this scheme can be particularly useful in order to achievetransmit diversity when signalling with higher order constellations(e.g. QAM, 8PSK or 16QAM) as the complexity of the equalizer is notincreased. If we attempt to achieve the same diversity by employingtransmit delay diversity, then the equalizer may become substantiallymore complex. This especially applies if the receiver uses an MLSE or asuboptimal version thereof.

Since the method described above, which we here call time-reversalspace-time block coding, can handle intersymbol interferference, we cancombine it with the well known method of transmit delay diversity [6].We can divide a group of transmit antennas into two groups. Within therespective groups we can use transmit delay diversity. We then view thetwo groups as two different channels and apply time-reversal space-timeblock coding to them. We thereby double the initial diversity that thetransmit delay diversity achieved within each group.

Both the time-reversal space-time block coding alone and the combinationwith transmit delay diversity increases the diversity in thetransmission. This means that the receiver sees more, somewhatindependenatly, fading signals and the probabillity that they all willhave low power at the same time is reduced. With more diversity thereceived signal level will vary even less and even less received poweris required at the subscriber unit. This can be used to increase therange of the system or increase the capacity as less power can betransmitted from the base station, thus creating less interference andthus allowing more users in the system. The so called fading margin inthe transmission can then be reduced. This means that a lower mean poweris required at the subscriber unit. This lower required mean power caneither be used to increase the range of the transmission by keeping thetransmitted power unchanged or increase the capacity of system bylowering the transmitted power and thereby reducing the interferencesuch that more users can be allowed into the system. The lower requiredmean power can also be used to increase the data rate to the subscriberif different data rates are available.

The above described embodiments of the invention are, obviously, merelyillustrative implementations of the principles of the invention andvarious modifications and enhancements can be introduced by artisanswithout departing from the spirit and scope of this invention, which isembodied in the following claims.

REFERENCES

-   [1] S. M. Alamouti, “A simple transmit diversity technique for    wireless communications,” Journal of Selective Areas of    Communications, vol. 16, no. 8, pp. 1451-1458, October 1998.-   [2] S. M. Alamouti, “Transmitter diversity technique for wireless    communications,”, International patent application PCT/US98/17963.-   [3] E. Lindskog. Space-time processing and equalization for wireless    communications, PhD thesis, Uppsala University, Signals and Systems,    PO Box 528, 751 20 Uppsala, Sweden, 1999, See www.signal.uu.se.-   [4] E. Lindskog, “Multi-channel maximum likelihood sequence    estimation,” in Proceedings of the 47th IEEE Vehicular Technology    Conference, vol. 2, Phoenix, Arizona, USA, May 5-7 1997, pp.    715-719.-   [5] A. J. Viterbi, “Error bounds for convolutional codes and an    asymptotically optimum decoding algorithm,” IEEE Transactions on    Information Theory, vol. 13, pp. 260-269, April 1967.-   [6] A. Wittneben, “Base station modulation diversity for digital    simulcast,” in Proceedings of the 41st Vehicular Technology    Conference, 1991.

1. A method of transmitting a signal having a sequence of symbolsthrough at least one channel with intersymbol interference, comprisingthe steps of: dividing the sequence of symbols to form a plurality ofsymbol streams; and processing the plurality of symbol streams beforetransmitting each symbol stream through a channel, wherein processingthe plurality of symbol streams comprises time-reversing at least one ofthe symbol streams before transmitting the at least one of the processedsymbol streams.
 2. A method for receiving and processing signalstransmitted from a transmitter to a receiver, comprising the steps of:receiving a first symbol stream in a first block of a frame; receiving asecond symbol stream in a second block of the frame; time reversing andtaking the complex conjugate form of the second symbol stream in thesecond block; and filtering the first symbol stream in the first blockand the time reversed and complex conjugate form of the second symbolstream to form decoupled outputs.
 3. The method of claim 2 wherein thefirst and second symbol streams each comprises first and secondportions, the first portion of the first symbol stream depending on afirst symbol sequence d, (t) and a second portion of the first symbolstream depending on a second symbol sequence d₂(t), the first portion ofthe second symbol stream depending on d₂(t), the second portion of thesecond symbol stream depending on d, (t), and the step of filteringfurther comprises filtering the first symbol stream and the timereversed complex conjugate of the second symbol stream in the secondblock using a matched filter according to $\begin{bmatrix}{z_{1}(t)} \\{z_{2}(t)}\end{bmatrix} = {\begin{bmatrix}{h_{1}^{*}(q)} & {h_{2}\left( q^{- 1} \right)} \\{h_{2}^{*}(q)} & {- {h_{1}\left( q^{- 1} \right)}}\end{bmatrix}\begin{bmatrix}{r_{1}(t)} \\{r_{2}(t)}\end{bmatrix}}$ wherein r₁(t) is the first symbol stream and r₂(t) isthe time reversed complex conjugate of the second symbol stream, z₁(t)and z₂(t) are the decoupled outputs, h₁(q⁻¹) is a polynomial in a unitdelay operator q⁻¹, describing a first channel from which the firstportion of the first symbol stream is received, h₂(q⁻¹) is a polynomialin the unit delay operator q⁻¹, describing a second channel from whichthe second portion of the first symbol stream is received, h*₁(q) andh*₂ (q) are polynomials in a unit advance operator q representingeffective channels from which the first and second portions of thesecond symbol stream are received, respectively, outputs z₁(t) and z₂(t)being decoupled in that z₁(t) depends on the first symbol stream d₁(t)and not on the second symbol stream d₂(t), and z₂(t) depends on thesecond symbol stream d₂(t) and not on the first symbol stream d₁(t). 4.The method of claim 2 further comprising the step of: after the step offiltering, estimating the symbol stream d₁(t) from output z₁(t) andsymbol stream d₂(t) from output z₂(t).
 5. The method of claim 2 whereeach of the first and second symbol streams is received by multipleantennas and is combined in order to increase signal quality and reduceinterference.
 6. A system for receiving and processing data comprising:at least one antenna adapted to receive a first symbol stream in a firstblock of a frame and a second symbol stream in a second block of theframe, each symbol stream comprising a plurality of symbols; a combiningfilter coupled to the antenna and adapted for time reversing and takingthe complex conjugate form of the second symbol stream received in thesecond block; and a matched filter coupled to the combining filter andadapted to form decoupled first and second outputs from the first symbolstream and the time reversed and complex conjugate form of the secondsymbol stream.
 7. The system of claim 6 further comprising an equalizeradapted to resolve intersymbol interference in the first and secondblocks.
 8. The system of claim 6 wherein the first and second symbolstreams each comprises first and second portions, the first portion ofthe first symbol stream depending on a first symbol sequence d₁(t) and asecond portion of the first symbol stream depending on a second symbolsequence d₂(t), the first portion of the second symbol stream dependingon d₂(t), the second portion of the second symbol stream depending ond₁(t), and the matched filter forms the decoupled first and secondoutputs according to $\begin{bmatrix}{z_{1}(t)} \\{z_{2}(t)}\end{bmatrix} = {\begin{bmatrix}{h_{1}^{*}(q)} & {h_{2}\left( q^{- 1} \right)} \\{h_{2}^{*}(q)} & {- {h_{1}\left( q^{- 1} \right)}}\end{bmatrix}\begin{bmatrix}{r_{1}(t)} \\{r_{2}(t)}\end{bmatrix}}$ wherein r₁(t) is the first symbol stream and r₂ (t) isthe time reversed complex conjugate of the second symbol stream, z₁(t)and z₂(t) are the decoupled first and second outputs, respectively,h₁(q⁻¹) is a polynomial in a unit delay operator q⁻¹, describing a firstchannel from which the first portion of the first symbol stream isreceived, h₂(q⁻¹) is a polynomial in the unit delay operator q⁻¹,describing a second channel from which the second portion of the firstsymbol stream is received, h*₁(q) and h*₂(q) are polynomials in a unitadvance operator q representing effective channels from which the firstand second portions of the second symbol streams are received,respectively. outputs z₁(t) and z₂(t) are decoupled in that z₁(t)depends on the first symbol stream d₁(t) and not on the second symbolstream d₂(t), and z₂(t) depends on d₂(t) and not on d₁(t).
 9. The systemof claim 8 further comprising: an estimator adapted to estimating thefirst symbol stream d₁(t) and the second symbol stream d₂(t) from thedecoupled outputs z₁(t) and z₂(t), respectively.
 10. The system of claim6 wherein each of the first and second symbol streams is received bymultiple antennas and is combined in order to increase signal qualityand reduce interference.
 11. The system of claim 6 further comprising anequalizer adapted to resolve intersymbol interference in the first andsecond blocks.
 12. A method for receiving and processing signalstransmitted from a transmitter to a receiver, the method comprisingreceiving a plurality of received symbol sequences each comprisingsymbols from a plurality of transmitter symbol streams, and processingthe received symbol sequences to generate decoupled outputs each forseparately detecting a different one of the transmitter symbol streams,wherein processing the received symbol sequences comprises timereversing at least one of the received symbol sequences after receivingit to generate at least one time reversed receiver symbol sequence. 13.The method of claim 12 wherein processing the received symbol sequencesfurther comprises complex conjugating at least one of the symbolsequences, and filtering at least one symbol sequence in its receivedform and at least one symbol sequence in a time-reversed and complexconjugated form to generate the decoupled outputs.
 14. The method ofclaim 12 wherein the plurality of symbol sequences are received from oneor more channels and comprise known symbols, the method furthercomprising estimating the one or more channels using the known symbols.15. The method of claim 1 wherein processing the plurality of symbolsstreams further comprises complex conjugating at least one of the symbolstreams before transmitting the at least one of the processed symbolstreams.
 16. The method of claim 3 wherein the first and second symbolstreams comprise known symbols, the method further comprising estimatingthe first and the second channels using the known symbols.
 17. Thesystem of claim 8 wherein the first and second symbol streams compriseknown symbols, the system further comprising a channel estimator adaptedto estimate the first and the second channels using the known symbols.18. An apparatus for receiving and processing signals transmitted from atransmitter, comprising: means for receiving a plurality of symbolsequences, each symbol sequence comprising symbols from a plurality ofsymbol streams; and means for processing the received symbol sequencesto generate decoupled outputs each for separately detecting a differentone of the symbol streams, wherein the means for processing the receivedsymbol sequences comprises means for time reversing at least one of thesymbol sequences.
 19. The apparatus of claim 18 wherein the means forprocessing the received symbol sequences further comprises means forforming complex-conjugated forms of at least one of the symbolsequences.
 20. The apparatus of claim 19 wherein the means forprocessing the received symbol sequences further comprises means forfiltering at least one symbol sequence in its received form and at leastone symbol sequence in a time-reversed and complex conjugated form togenerate the decoupled outputs, each decoupled output depending on adifferent one of the symbol streams.